28G THE rrvINCirLES OF [Paut II 



mul an iininciisc crest cliaractcrizc tlic adults of botli 

 sexes. 



Finally, from what wc have now seen of t-lic relation 

 Avhich exists in many natural species and domesticated 

 races, between the period of the development of their 

 characters and the manner of their transmission — for ex- 

 ample, the striking fact of the early growth of the horns 

 in tlie reindeer, in which both sexes have horns, in com- 

 liarison with their much later grow^th in the other species 

 in which the male alone bears horns — we may conclude 

 that one cause, though not the sole cause, of characters 

 being exclusively inherited by one sex, is their develop- 

 ment at a late age. And secondly, that one, though ap- 

 ])arently a less efficient, cause of characters being in- 

 herited by both sexes is their development at an early 

 age, while the sexes differ but little in constitution. It 

 appears, however, that some difference must exist between 

 the sexes even during an early embryonic period, for char- 

 acters developed at this age not rarely become attached 

 to one sex. 



Summary and conclucling remarJcs, — From the fore- 

 going discussion on the various laAVS of inheritance, we 

 learn that characters often or even generally tend to be- 

 come developed in the same sex, at the same age, and pe- 

 riodically at the same season of the year, in which they 

 first appeared in the parents. But these laws, from un- 

 known causes, are very liable to change. Hence the suc- 

 cessive stcjis in the modification of a species might readily 

 be transmitted in different ways ; some of the steps being 

 transmitted to one sex, and some to both ; some to the 

 oflspring at one age, and some at all ages. Not only arc 



dcr Domestication,' vol. i. pp. 250, 256. In regard to the higher ani- 

 mals, the sexual difTercnces ^Yllich have arisen under domestication arc 

 described in the same work under the head of each species. 



